Substantial earth movements along clay-bearing bedding planes have produced landslides in road cuts along the Los Angeles-San Francisco superhighway, Interstate 5, where it crosses the western extension of the San Gabriel Mountains in southern California between Castaic and Gorman. Such slides occur where the strike of the clay-bearing strata in road cuts is parallel with the highway, and the dip is toward the roadway at an angle less than the slope of the cut. Slides also occur on dip slopes.

Thick sandstone beds have moved on wet clay-bearing members that range from a few inches to as much as 20 ft thick. Laboratory studies show that slide-prone montmorillonite and illite are abundant minerals in the clay-bearing strata. Further, when the water content of the clay that contains these minerals exceeds 50 percent, the cone shear resistance drops to 0.001 to 0.1 tsf, whereas at 20-35 percent water content, the shear remains in excess of 1.0 tsf.

Where the strike of strata is normal to the direction of the highway, landslides either have not been observed, or are minor features. In places where the dip is away from the roadbed, although the strike is parallel with the highway, slides ordinarily are not observed.

A monumental highway fill across West Liebre Gulch merits special attention. South of the gulch, strata strike approximately normal to the line of the freeway and dip north toward the bottom of the gulch. Clay-bearing zones lie just beneath the surface. Apparently these caused an old landslide, formed long before construction was contemplated, which extended across the line chosen for the freeway. Fill, emplaced when the freeway was constructed, suffered extensive slide action when the old slide, partly buried beneath the fill, was reactivated following heavy rains both in 1967 and in 1969.